1. Field of the Invention
This invention relates generally to electronic test equipment and more specifically to signal analyzers for measuring harmonic distortion. The invention also relates to measuring particular characteristics of an input signal such as the relative amplitudes of its fundamental and other individual harmonics.
2. Description of the Prior Art
Various analyzers have been used in the past to measure the amount of harmonic distortion present in an input signal. For example, wave analyzers which function as frequency selective volt meters have been used to measure the relative amplitude of any frequency in the passband of the wave analyzer.
In a wave analyzer a local oscillator modulates the incoming signal to provide a summation or difference frequency the voltage of which is measured to provide an indication of signal magnitude. Using a wave analyzer, the fundamental frequency of the input signal can be determined and the magnitude of this frequency component can be measured. By doubling or tripling the fundamental frequency in the local oscillator of the wave analyzer, the magnitude of the respective second or third harmonic in the input signal can be measured. In some wave analyzers, the sensitivity of the meter has been adjustable to provide a full scale or 100 per cent reading for the magnitude of the fundamental frequency. This has enabled the meter to be calibrated to provide a percentage indication of the amount of distortion associated with a particular harmonic component.
Distortion analyzers have also been used to measure the amount of total harmonic distortion plus noise which is present in an input signal. In a distortion analyzer, the magnitude of the fundamental frequency is not measured since this frequency is rejected. Although the distortion analyzer measures the total harmonic distortion present in a signal, it does not provide an indication of the amount of distortion caused by a particular harmonic component. In many cases, adjustments needed to compensate for second harmonic distortion are different than those needed to compensate for third harmonic distortion. Since the distortion analyzers have not been capable of distinguishing these harmonics, they have not performed satisfactorily under these circumstances.
A wave analyzer is typically used in conjunction with other types of electronic measurement apparatus to determine the amount of harmonic distortion produced by a test unit, such as a magnetic recorder. Such a combination of measurement apparatus would typically include a main oscillator and a voltmeter connected to the input of the recorder, and a counter and wave analyzer connected to the output of the recorder.
After the recorder has been set up and its output level adjusted to some reference point characteristic of that recorder, the wave analyzer is calibrated to the full scale reading at that level. The wave analyzer is initially manually adjusted to provide a maximum output signal at a particular frequency. This has been the method for determining the magnitude of fundamental frequency components of the input signal. In some cases, the counter has been used to provide an estimate of the fundamental frequency. Although this has reduced the time needed to locate the exact fundamental frequency the manual procedure for locating the exact fundamental frequency has been quite time consuming.
After the fundamental frequency has been determined, the wave analyzer can then be tuned to the particular harmonic frequency whose magnitude is to be measured. By adjusting the sensitivity of the wave analyzer's meter, a reading of the harmonic frequency has provided a measure of the percentage of distortion.
In some apparatus, the main oscillator, the wave analyzer, and the voltmeter are provided in a single unit. A separate switch is used so that the voltmeter can measure the magnitude of both the signal provided by the main oscillator and the signal provided at the output of the test unit.
In some devices the tuning capacitors of the main oscillator are ganged with the tuning capacitors of the local oscillator in the wave analyzer. In this combination, the manual tuning of the main oscillator also results in the manual tuning of the local oscillator of the wave analyzer.
The use of any of these apparatus to test a unit such as a recorder has been time consuming even to provide an initial indication of the amount of distortion present in a signal. They have been particularly unsatisfactory if the degree of that distortion has been greater than desired, typically one per cent of the fundamental frequency. Under these circumstances, it has been desirable to adjust the record level of the recorder and also to readjust the reproduce level of the recorder to provide the original output level. Unfortunately, the readjustment of the reproduce level changes the level of the fundamental frequency as well as the level of all the harmonic components of the input signal. As a consequence, the fundamental frequency must be relocated to recalibrate the wave analyzer to a particular signal level. Only after recalibration can the wave analyzer be used to remeasure the percentage of harmonic distortion. In some cases the record level must be subsequently readjusted so that this cumbersome procedure must be repeated several times.